The present exemplary embodiment relates to High Intensity Discharge (HID) lamp lighting systems. It finds particular application in conjunction with metal halide lamp dimming systems and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications, including mercury lamps and high pressure sodium (HPS) lamps.
In general, HID lamps suffer from a degradation in light output over time. This degradation in light is commonly referred to as the Lamp Lumen Depreciation (LLD) of a lamp. The LLD of a lamp is defined as the light output vs. time. When MH lamps are operated in the fully dimmed mode, the metal halide vapor pressures drop by a very large amount and the lamp reverts to a mercury discharge. A mercury discharge under these conditions will have a very poor Color Rendition Index (CRI), a low efficiency and poor LLD characteristics.
There is widespread evidence that the LLD during full power operation can be dramatically improved through the use of frequencies higher than 60 Hz. The two frequency domains that are commonly used for this are approximately 100 Hz square waves and higher frequencies of 100 to 200 KHz for 400 W lamps.